Spinal connection systems may be used in orthopedic surgery to align and/or fix a desired relationship between adjacent vertebrae. Such systems typically include a spinal connection element, such as a relatively rigid fixation rod or plate or a dynamic connector, that is coupled to adjacent vertebrae by attaching the element to various anchoring devices, such as hooks, bolts, wires, or screws. The spinal connection element can have a predetermined contour that has been designed according to the properties of the target implantation site, and once installed, the spinal connection element holds the vertebrae in a desired spatial relationship, either until desired healing or spinal fusion has taken place, or for some longer period of time.
Spinal connection elements can be anchored to specific portions of the vertebra. Since each vertebra varies in shape and size, a variety of anchoring devices have been developed to facilitate engagement of a particular portion of the bone. Pedicle screw assemblies, for example, have a shape and size that is configured to engage pedicle bone. Such screws typically include a threaded shank that is adapted to be threaded into a vertebra, and a head portion having a spinal connection element receiving portion, which, in spinal rod applications, is usually in the form of a U-shaped slot formed in the head for receiving the rod. A set-screw, plug, cap or similar type of closure mechanism, may be used to lock the connection element into the connection element receiving portion of the pedicle screw. In use, the shank portion of each screw may be threaded into a vertebra, and once properly positioned, a connection element may be seated through the spinal connection element receiving portion of each screw and the connection element is locked in place by tightening a cap or similar type of closure mechanism to securely interconnect each screw and the connection element. Other anchoring devices also include hooks and other types of bone screws.
In certain procedures, such as those in the lumbar or sacral spine, it may be necessary to use a larger diameter pedicle screw capable of carrying large loads or engaging large pedicles. A difficulty in using a larger diameter screw comes from the corresponding increase in the size of the receiver head to accommodate the larger diameter screw shank, since the shank is usually assembled from the top through the opening at the proximal end of the receiver head. The increased size of the receiver head can interfere with the bony anatomy and can limit the polyaxial range of motion of the screw head. Another problem associated with manufacturing large diameter top-loading screws is that the opening in the receiver head has to be larger to accept the larger diameter screw shank, which creates the need for a larger closure mechanism. It is desirable to maintain the same size opening in the receiver head such that the same size closure mechanisms can be used. Accordingly, a larger diameter polyaxial screw is needed which is not top-loading.